This invention relates to the soldering of printed circuit boards or the like in which the boards pass over a wave soldering section.
A wave soldering machine is a machine for soldering components pre-mounted on a printed circuit board to that board. The board is held with components on the board having leads projecting down through holes in the board. Sometimes small components (chips) may be cemented to the bottom surface of the board. The board is passed over the top of a solder wave to just skim its surface. During this passage, strong, conductive solder joints have to be formed between the components and the board. A well soldered board has the following joint characteristics.
(1) All exposed copper of the board and component leads are well wetted.
(2) All joints are continuous.
(3) Solder has flowed through the holes in the PCB, partly by capillary action, to make nice through connections to the top surface of the board.
(4) There is no bridging between component leads, pads or tracks on the PCB causing a false circuit.
(5) There are no icicles of frozen solder hanging from the bottom of the board.
(6) None of the components have been damaged by the temperature of the process.
(7) There is no excessive alloying between the leads, or copper of the board and the solder.
(8) There are no inclusions or impurities in the solder.
Some of these qualities are affected by the temperature of the board when it meets the solder wave and to overcome this, the board is preferably heated prior to soldering. Another feature of these qualities is to do with the length of time the components of the board remain in the solder wave and this can partly be achieved by varying the configuration of the solder wave as well as heating the solder to the required temperature. The heating of these components has to achieve a balance between damage by thermal shock and high temperature to the board and components or excessive soldering time required to transfer heat from the solder in the solder wave to the board, thus the speed of the board passing over the solder wave also affects the configuration of the solder wave.